the easiest way to immobilise your car is to put a switch in the coil lead that runs to the engine.
This will stop the car from being started unless it is switched on.
The coil lead runs with all the others next to the passengers sill (wire can be identified in the Haynes manual).
I cut it about half way down the sill.
I used the brown cable cut from a standard house appliance.
(while this isn't very scientific it will allow more current than it will ever need so it is safe. alternatively buy about 2Metres of 15amp cable from B&Q)
When you extend the coil lead you will be able to fit it wherever you chose. The better hidden it is the better.
One possibility is to replace one of the blank dashboard switches for a real one and run the cable to there. or buy a switch like this one
http://www.maplin.co.uk/Module.aspx?ModuleNo=2353&C=Maplin&U=SearchTop&T=push%20switch&doy=23m3Then simply switch off when you leave the car and switch on again when you want to drive away.